This application claims the benefit of a Japanese Patent Application No. 2002-380665 filed Dec. 27, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to optical recording media and recording methods therefor, and more particularly to a so-called rewritable phase change type (or phase transition type) optical recording medium which may be used to record information thereon and reproduce information therefrom by irradiating a light beam to cause an optical change (or transition) in a recording layer material thereof, and to a recording method suited for recording information on such a phase change type optical recording medium at a high speed.
2. Description of the Related Art
Recently, recording speeds for phase change type optical recording media such as CD-RW, DVD+RW and DVD-RW have improved, and high-speed recording systems are being developed to realize a 24-times speed recording in the case of the CD-RW and to realize a 2-times to 4-times speed recording in the case of the DVD. With respect to a rewritable phase change type optical recording medium employing the DVD format, there are active research and development to realize an optical recording system aimed at recording information at even higher recording speeds.
Formats of conventional recording strategies usually set a basic (or fundamental) period to 1T, where T denotes a channel period. The “channel” indicates a time or spatial length per data bit, and is standardized for the DVD, for example. But as the recording speed increases and the recording is made by setting a recording pulse width of a light beam to be the same as the conventional channel period, there was a problem in that it is impossible to obtain a sufficiently high power when recording an amorphous mark. In order to solve this problem, a Japanese Laid-Open Patent Application No. 2001-331936 proposed a method of setting the recording strategy such that the basic period is 2T.
In other words, according to the setting of the conventional recording strategy, the number of recording pulses of the light beam is set to n-1 or b-2 when a length of the recording mark is denoted by nT. For this reason, the recording is prescribed such that the number of recording pulses is increased every time the length of the recording mark increases by 1T. In the case of the recording strategy employing the basic period of 2T, the number of recording pulses increased every time the length of the recording mark increases by 2T, so as to reduce the increase in the number of recording pulses and reduce jitter even in the case of a high-speed recording.
However, the recording strategy employing the basic period of 2T does not sufficiently take into consideration a relationship between a rise and fall characteristic of a light beam power of a recording apparatus used for the recording. For this reason, the recording strategy employing the basic period of 2T is not always satisfactory, and there was a problem in that the setting of the recording pulses is not always appropriate.
For example, the above described method proposed in the Japanese Laid-Open Patent Application No. 2001-331936 can be applied to a high-speed recording such as the 4-times speed or higher in the case of the rewritable-DVD. But according to experiments conducted by the present inventors, it was found desirable that an application limit of the above described method is on the order of 6-times speed of a reference recording speed (1-times speed: 3.49 m/sec) of the DVD.
In addition, when forming the recording mark by the conventional recording method at a higher speed, such as at a 10-times speed of the recording speed of the DVD, the pulse width needs to be set to approximately 1.9 nsec if a duty ratio is set to 0.5 when forming the recording mark, as shown in FIG. 1. FIG. 1 is a diagram showing the recording pulses used by the conventional recording method. In FIG. 1 and FIGS. 2 and 3 which will be described later, the ordinate indicates the recording pulse amplitude (or power level), and the abscissa indicates the time. In FIG. 1, Pw denotes a recording (write) power, Pe denotes an erasing power, Pb denotes a bottom power, and W denotes the width of the recording pulse.
On the other hand, rising and falling time constants of the recording pulses which can be formed by direct modulation of an existing semiconductor laser diode (LD) are approximately 2 nsec, as shown in FIG. 2. FIG. 2 is a diagram showing rising and falling edges of the recording pulses which can be formed by direct modulation of the existing semiconductor laser diode (LD). Hence, it is impossible to emit a light beam having the required recording power Pw (or power level) within an irradiation time. In FIG. 2, Pr denotes a rising edge of the recording pulse, and Pf denotes a falling edge of the recording pulse.
For example, even when carrying out a 10-times speed recording according to the recording strategy which employs the basic period of 2T, the actual recording pulse width having the duty ratio of 0.5 is 3.8 nsec, and although such a recording pulse can be formed, the half breadth becomes extremely narrow as shown in FIG. 3. FIG. 3 is a diagram showing the half breadth of the actual recording pulses used by the conventional recording method. Consequently, it may be easily seen from FIG. 3 that it is difficult to obtain a sufficiently high power for forming the amorphous mark. In FIG. 3, Phb denotes the half breadth of the recording pulse.
Therefore, there are demands to realize a further improved recording strategy which can cope with the high-speed recording, such as the recording speed on the order of the 6-times speed or higher in the case of the DVD, and is capable of obtaining a sufficiently high power when forming the amorphous mark even when the recording speed increases.